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Scent of fear puts brain in emergency mode

The smell of the sweat you produce when terrified is not only registered by the brains of others, but changes their behaviour too, according to new research. It adds to a growing body of evidence that humans may communicate using scent in a similar way to how other animals use pheromones.

The smell of their sweat was wafted under the noses of volunteers as they lay in an fMRI scanner. Even though they had no idea what they were inhaling, two separate sets of volunteers showed activation of the amygdala – the area of the brain responsible for emotion-processing, plus areas involved in vision, motor control and goal-directed behaviour. Sweat produced under non-stressed conditions didn’t produce this reaction.

What’s more, in behavioural tests, the “stress sweat” seemed to heighten people’s awareness of threat, making them 43 per cent more accurate in judging whether a face was neutral or threatening.

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Ethical issues

Because the study used sweat rather than its components, this is not definitive evidence that human pheromones exist, says Johan Lundstrom, a pheromone researcher at Monell Chemical Senses Center in Philadelphia, who was not involved in the research.

“I’m not naïve about the fact that some people will look at this study and say it was irresponsible,” says Mujica-Parodi. There are obvious ethical issues about synthesising a chemical that could induce fear in other people, and the group’s early research was funded by the US military (pdf).

But Mujica-Parodi insists that the chemical, if shown to have the same effects as full sweat, could be put to all manner of non-sinister uses, such as understanding the dynamics of fear in situations where people are thrown together in confined spaces, like aeroplanes, jails, or submarines.

Stress tool

It could also make training for stressful jobs – such as soldiers, pilots and surgeons – more realistic. And since it seems to heighten awareness and vigilance, it could be used as a stimulant&colon; to maintain alertness on long car journeys, for example.

Simon Wessely, a psychiatrist at the King’s Centre for Military Health Research at King’s College London, says that even if the chemical was to be unleashed in a crowd, it would be highly unlikely to cause mass panic.

“Fear is biological, of course, but the important thing is the psychology and how you cognitively appraise the situation,” he says. “I don’t think you’d get terrified for no obvious reason.”